The present invention relates to a method of calculating a value of at least one of a tire axial force and self-aligning at the time when a slip ratio is given to a tire, a method of deriving a value of a tire dynamic element parameter used in a tire dynamic model from a characteristic curve expressing dependency of at least one of the tire axial force and a self-aligning torque on the slip ratio, a vehicle traveling simulation method of performing a simulation of traveling of a vehicle fitted with the tire using the characteristic curve expressing the slip ratio dependency, and a method of designing the vehicle or the tire by performing the vehicle traveling simulation using information of the characteristic curve expressing the slip ratio dependency and information of a vehicle specification.
Nowadays, a vehicle specification that realizes desired vehicle performance is determined in an initial stage of vehicle development by performing a computer-based simulation and judging whether the desired vehicle performance is achieved. In other words, a vehicle model is created based on data of a set vehicle specification, a vehicle traveling simulation is performed by giving a tire generation force to this vehicle model, and performance of a vehicle under this vehicle specification is evaluated based on a result of the simulation.
In particular, when a vehicle traveling simulation is performed, tires are sole construction components that transfer a force received from a road surface to a vehicle and are components that give a significant influence on performance of the vehicle. Therefore, there is a necessity to determine a tire generation force accurately. The generation force described in this specification refers to a longitudinal force and a lateral force during cornering. A technique is proposed with which a characteristic curve expressing dependency of a lateral force of a tire, which occurs during cornering as the tire generation force, and a self-aligning torque on a slip angle, a characteristic curve of a longitudinal force that occurs during braking/driving, and characteristic curves of the longitudinal force, the lateral force, and the self-aligning torque that occur during the braking/driving and the cornering are approximated with “Magic Formula” represented in a standard form by formula (20) given below.Y(x)=D sin [C tan−1{Bx−E(Bx−tan−1(Bx))}]  (20)
Here, “Magic Formula” is a non-analytic model using a nonlinear approximation expressing a tire characteristic by determining values of respective parameters B to E in formula (20). In this model, as to a lateral force Fy in the case where a composite characteristic such as “a change of dependency of a lateral force on a slip angle according to a slip ratio S” is expressed, a lateral force Fy0 under a condition, in which no slip ratio S is applied as in the formula (20), is amended as expressed by formula (21) given below. A longitudinal force and a self-aligning torque are also amended in the same manner as in the case of the lateral force.Fy=Gyk·Fy0+Svyk  (21)
(Gyk is a coefficient associated with the slip ratio S and Svyk is a coefficient associated with the slip ratio S)
Generally, a vehicle traveling simulation is performed using values of the parameters B to E, Gyk, Svyk, and the like of this “Magic Formula” (hereinafter referred to as the “Magic Formula” parameter).
A conventional vehicle designing method using such a vehicle performance evaluation will be described.
First, in an initial stage of vehicle development, a vehicle model is created by setting a desired vehicle specification (such as a desired wheel base, height of a gravitational center, and weight distribution between vehicle front and rear wheels) and values of the “Magic Formula” parameters of a tire are set and given to the vehicle model. Next, a traveling condition for a vehicle traveling simulation is set and the vehicle traveling simulation is performed in accordance with this traveling condition. The traveling condition is changed in accordance with performance to be evaluated and when the performance evaluation is a durability evaluation, for instance, a vehicle traveling speed and profile data of roughness or the like of a road surface are set as the traveling condition. In the case of an emergency avoidance performance evaluation, a vehicle traveling speed, a steering angle, and braking/driving data according to a slip ratio including a slip ratio in a braking/driving direction, profile data of an actual road surface, or the like is set as the traveling condition.
Meanwhile, as examples of the vehicle model, it is possible to cite an analysis model for mechanism used by an analysis software for mechanism, such as “ADAMS” (manufactured by MDI Corporation in USA), and an analysis model used in vehicle movement analysis software combined with dedicated software for control and design such as “CarSim” (manufactured by Mechanical Simulation Corporation) or “MatLab” (manufactured by The MathWorks, Inc.).
Next, performance evaluation data that is a performance indicator is extracted based on a result of the traveling simulation. In the case of the durability performance, for instance, the maximum value of a stress placed on a specific member or a deformation amount of the specific member is used as the performance evaluation data. In the case of the emergency avoidance performance, stability based on a relation between the vehicle traveling speed and the maximum lateral acceleration or a vehicle slip angle and stability of the vehicle slip angle's changing rate due to the vehicle's tracing is evaluated. When the performance evaluation data does not reach a target value determined at a vehicle planning stage, a part of the vehicle specification is amended. For instance, the weight distribution between the vehicle front and rear wheels is amended, a characteristic of a construction component such as suspension is amended, or a tire generation force, such as a characteristic curve of a tire longitudinal force or a characteristic curve of a tire lateral force, is amended. When the tire generation force is amended, values of the “Magic Formula” parameters described above are amended.
Then, the amendment described above is repeated until the performance evaluation data reaches the target value. When the target value is reached, the vehicle specification, the construction component characteristic, or the values of the “Magic Formula” parameters expressing the tire characteristic at that time are determined as a requisite characteristic.
A car manufacturer proceeds to detailed designing of construction components in accordance with this determination. For tires that cannot be designed by the car manufacturer, the car manufacturer presents the determined tire requisite characteristic, in other words, the determined values of the “Magic Formula” parameters as requisite items to a tire manufacturer and orders tire delivery.
In response to this order, the tire manufacturer performs tire structure designing and tire material designing, in which the values of the “Magic Formula” parameters described above are realized, and repeats manufacturing a prototype of the tire so that the presented requisite items are satisfied.
With the method described above, however, the “Magic Formula” parameters used as the tire requisite characteristic are parameters of a model specialized on a description of a cornering characteristic in which no consideration is given to actual tire dynamical behavior. Therefore, the “Magic Formula” parameters expressed by formula (20) given above do not reflect a structural mechanism giving an influence on a tire cornering characteristic. This leads to a problem that even when the values of the “Magic Formula” parameters are given as the requisite items, the values are not linked with tire dynamic element parameters expressing various kinds of tire stiffness giving an influence on the tire cornering characteristic and are not directly reflected in the tire structure designing and material designing. Also, the “Magic Formula” parameters given as the requisite items are parameters determined without giving consideration to the tire structural dynamic mechanism, so there are many cases where it is impossible to say that the parameters are realizable with an actual tire.
Meanwhile, JP 2002-356106 A discloses a tire designing method with which it is possible to perform optimal designing by evaluating performance of a tire through combination with a vehicle fitted with the tire.
In brief, in JP 2002-356106 A, the tire is modeled with finite elements based on a design value, a tire traveling simulation is performed, tire characteristic data is acquired through this simulation, and a vehicle traveling simulation is performed using this tire characteristic data. With this method, the tire is finely modeled with the finite elements, so there are an extremely large number of parts that are amendment targets, which results in occurrence of many cases, in which it is unclear which part should be amended, and which results in a necessity to rely on a skilled designer. Also, there is a necessity to repeat the tire traveling simulation and the vehicle traveling simulation many times, which makes it difficult to efficiently find a design value that realizes satisfactory performance.
In addition, with “Magic Formula”, as described in “TIRE AND VEHICLE DYNAMICS” (HANS. B. PACEJKA, ISBH 0-7506-5141-5), pp. 172-191, it is possible to amend a lateral force and a longitudinal force which occur to a tire, in other words, it is possible to amend a tire characteristic by changing a scale factor λ. In other words, it is possible to amend the lateral force and the longitudinal force which occur to the tire independently of each other. For instance, in a development initial stage, it is possible to make a setting so that the tire longitudinal force is increased to thereby significantly enhance stop distance performance, make a setting so that the lateral force is decreased to thereby prevent a turnover at the time of emergency obstacle avoidance, and determine them as a tire requisite characteristic.
The lateral force and the longitudinal force that occur to the tire, however, are not forces that occur due to independent mechanisms from the viewpoint of structural mechanics but are closely related forces, so there are also many cases where the tire demand characteristic described above is unrealizable with an actual tire.
When the contents of JP 2002-356106 A are described in a more concrete manner, first, modeling is performed with tire finite elements based on input of a tire design value and a tire traveling simulation is performed. Tire characteristic data is calculated through this traveling simulation and the “Magic Formula” parameters are obtained from this tire characteristic data.
On the other hand, a vehicle model that is a mechanism analytic model is created based on a setting of data of a vehicle specification. Next, a traveling simulation condition is set and given to the vehicle model together with the “Magic Formula” parameters obtained in advance, and a vehicle traveling simulation is performed. Performance evaluation data is calculated from a result of the traveling simulation.
Generally, for instance, in the case of durability performance, the maximum value of a stress placed on a specific member or a deformation amount of the specific member is used as the performance evaluation data. In the case of emergency avoidance performance, stability based on a relation between a vehicle traveling speed and the maximum lateral acceleration or a vehicle slip angle and stability of the vehicle slip angle's changing rate due to the vehicle's tracing is evaluated.
When this performance evaluation data does not reach a target value, it is judged that the target is not achieved and the tire design value is amended. In accordance with this amendment, the tire model constructed with the finite element is created again.
Then, the tire design value is repeatedly amended until the performance evaluation data obtained as a result of the vehicle traveling simulation reaches the target value. When the performance evaluation data reaches the target value, the amended tire design value is determined as a tire design specification.
With the designing method described above, however, when it is impossible to achieve the predetermined target performance in the performance evaluation, the tire design value (such as a tire shape or a size of each tire part) is amended, so it is difficult to find how the “Magic Formula” parameters exerting a direct influence on the vehicle traveling simulation is to be amended through a result of the amendment of the tire design value and it is impossible to sufficiently grasp whether the target performance is approached through the amendment of the tire design values. Therefore, in order to efficiently determine the tire design specification by amending the tire design values, the work needs to be done by a skilled designer, meaning that it is impossible for everybody to determine the design specification with ease. In other words, the tire is finely modeled with the finite elements, so there are an extremely large number of parts that are amendment targets, which results in occurrence of many cases where it is unclear which part should be amended and makes it unavoidable to rely on a skilled designer. Also, there is complexity in which each time tire design values are amended, the tire model that is a finite element model needs to be created again. Further, it is required to repeat the time-consuming tire traveling simulation and vehicle traveling simulation many times, which makes it difficult to efficiently find tire design values that realize satisfactory performance. In particular, in a vehicle traveling simulation in which a slip angle and a slip ratio in a braking/driving direction are given to a tire, it is difficult to find such a tire design value with efficiency.